The present invention relates to a picture capturing apparatus and to a method of capturing a picture.
Miniaturization of camera modules and reduction of their manufacturing cost is an important goal in many fields, such as the field of mobile phone cameras and the field of automotive engineering. In all dimensions, the module size is proportional to the pixel size. This also applies to the length along the optical axis: relatively small pixels reduce the sensor size for a given overall number of pixels; a relatively small sensor enables a shorter focal length with a given field of view. The minimum extension along the optical axis of a camera module is limited mainly by the focal length of the optical system.
Inspired by compound eyes of insects, multi-channel imaging systems promise progress in both aspects, namely miniaturization and cost reduction. Instead of a single stack of lenses with an extension in the order of magnitude of some millimeters, the optical system in multi-channel imaging systems consists of an array of microlenses. The micropictures generated by these microlens arrays are optically or electronically combined into a single picture. With a specific predefined field of view of the overall system, the focal length of the microlenses of such a multi-channel imaging system is only a fraction of the focal length of a single-channel system, whereby the design height along the optical axis is reduced, and the focal depth is significantly increased. Since each micropicture covers only a small portion of the field of view, the optical systems may be configured to be simple: frequently, a lens and a few apertures per channel are sufficient. Since the microlenses have diameters in the order of magnitude of hundreds of micrometers, and a vertex height in the range of several 10 micrometers, they may be produced cost-effectively in a wafer bond arrangement and with a high precision.
However, so far it has been a challenge to achieve sufficient resolutions for applications such as for cameras, mobile phones or the like. In spite of the relatively axially parallel optical path of each individual channel, optical distortions occur for each channel, which complicates stitching together of the individual partial pictures on the individual imaging channels. If the attempt is made to mutually align microlens arrays and associated subareas of the image sensor with such precision that the frame arrays of the subareas of the image sensor need only be stitched together to form an overall pixel array, this has meant large requirements placed upon manufacturing tolerances and upon tolerances in the assembly of image sensor and microlens array. In Brückner, Duparré, Dannberg, Leitel and Bräuer: “Driving Microoptical Image System towards Miniature Camera Applications, Micro-Optic 2010, Proceedings of SPIE, vol. 7716, an algorithm for combining partial pictures of a multi-channel imaging system to form an overall picture is described, which is effected in several stages: The micropictures are initially inverted and subsequently equalized, whereupon the micropictures are joined together to form an overall picture by being resorted while taking into account the parallax. Equalization is performed with subpixel accuracy. The individual pixel clouds thus obtained are subject to interpolation so as to create partial pictures that are equalized in each case and wherein the picture information is specified to integer pixel positions. Said partial pictures are finally interwoven to form an overall picture by rearranging the pixels or by interleaving the pixels in the overlap areas of the equalized partial pictures. Even though the approach disclosed there reduces the above problem of the large requirements placed on the manufacturing accuracies, there remains the problem that due to mismatches of individual processing of the partial pictures, artifacts may arise in the overall picture produced, in particular along the overlap areas of the partial pictures.
Problems of a similar kind as were described above with reference to the multi-channel imaging systems for achieving a larger overall field of view also occur in other applications. US 2010/0013807 A1 entitled “Scene Independent Method for Image Formation in Lenslet Array Imagers” describes, e.g., an array of microlenses wherein the lenses have to face more or less the same direction and have to have the same image aperture angle. The viewing directions of the individual microlenses differ only to a very small extent, however. In particular, the patent describes how the differences in the viewing directions of the individual partial pictures may be determined. In this respect, a description is given, inter alia, of how the partial pictures may be placed one upon the other very accurately, said patent using a global shift. However, potential distortion of the frames is not compensated for.